I. Field of the Invention
This invention relates generally to the measurement of electrical energy consumption in a power system and more particularly to an improved method and apparatus for metering electrical energy consumption at multiple rates depending upon the time of day, time of week and time of year during which the energy is being consumed.
II. Description of the Prior Art
Initially, electrical energy had been sold quite generally on the basis of a fixed rate schedule irrespective of whether a high or low demand was made on the electrical generation system. Because of this flat rate structure, electrical generation and distribution systems were overloaded at certain times and, at other times, were only very slightly loaded. This resulted in substantial inefficiencies since a much larger plant and distribution system than is economically desirable is required in order to meet peak demands of the distribution system.
In order to provide for more efficient use of electrical generation and distribution facilities during off-peak load periods, attempts have been made at developing clock mechanisms for decreasing the flow of electricity during peak load periods. Generally, in the past, these types of devices have been found to be impractical due to the necessity of frequent checking and setting of the clock switching mechanisms at the various consumer locations and, in addition, to the inability to predict peak demand load intervals and to change the timing of the clock mechanisms on a relatively simple and efficient basis to track the peak load intervals.
Very early in the supply of electricity over a relatively large distribution area, attempts were made at multiple rate metering of electrical power consumption. These attempts were initiated in order to be able to charge a lower rate to those consuming electrical energy during the off-peak periods than the rate charged to those consuming the energy during on-peak periods. The theory behind these multiple rate structures is to encourage the user to consume the electrical energy during off-peak periods which would thereby tend to level the load requirements placed on the power distribution system.
As an example of a prior attempt to individually register different amounts of power consumed during different intervals of a day, Greenwood et al disclosed in U.S. Pat. No. 2,139,821, a simplified arrangement wherein a two-stage mechanical watt meter mechanism was provided having a clock timer associated therewith. Cams on the clock timer tripped a mechanism which, at selected times during the day, permitted the driving of one set of meter dials or the other so that consumption of electrical energy, during a selected portion of the day, could be registered by only one set of dials, while the electrical energy consumed during the other portion of the day could be registered on a second set of dials.
This arrangement was followed by the developments of Pratt, as disclosed in U.S. Pat. No. 2,246,185 and Cameron, as disclosed in U.S. Pat. No. 2,132,256. The Pratt and Cameron developments both utilized dual rate metering systems wherein a clock determined which set of meters were to be activated. Other attempts have been made at providing a dual rate kilowatt hour metering system, as exemplified by the following U.S. Pat. Nos.: Barstow, 548,419; Cox, 585,258; Oxley, 591,195; Oxley, 593,852; Wilson, 596,283; Kapp, 597,985; Oxley, 710,070; Mayer, 1,078,206; Ogurkowski, 2,415,653; Kahn, 2,915,704; and Staker, 4,082,999.
An improved apparatus for metering the consumption of electrical energy at multiple rates is disclosed by Germer, et al in U.S. Pat. No. 4,050,020, which patent is assigned to the assignee of the present invention. This apparatus employs three sets of decade gear driven dials for registering kilowatt hours consumed on a continuous basis, kilowatt hours consumed during preselected mid-peak intervals, and kilowatt hours consumed during preselected high-peak intervals. A programmable control circuit controls the operation of the alternate sets of decade gear driven dials by actuating the desired set at a predetermined time. Although an improvement over its predecessors, this apparatus is limited in its versatility since its time interval selection is based on time which is internally generated in 15-minute increments on a weekly cycle.
In addition to the aforementioned two-rate mechanical registering systems, other systems have developed wherein the use of power above a certain level is recorded. These systems include electronic watt-hour meters which are capable of measuring a maximum peak power over a predetermined time period. In order to permit power companies to supply a level load, and also to have generating capability to supply all consumers at all times, it is necessary that peak loads be predictable. Thus, there is a practice of billing large users in particular, based on a peak load within a given period of time, typically 15 minutes. This gives the user an incentive to maintain a level load and not exceed a predetermined peak. This monitoring of excess consumption is a proposed means of reducing the peak use of power by a particular customer. Such systems minimize individual peak power usage which may or may not coincide with the peak power demand on a utility system.
In a time-of-day demand metering system, the consumption of power is measured and recorded on a demand meter register during demand intervals of a predetermined length. Previously, the demand interval length had been for a fixed length of time (e.g. 15 or 30 minutes). However, as the costs of generating electrical energy escalated and the variety of demand power users increased (the demand for power during on-peak periods can be greater and for longer periods in some areas than in others) utility companies required a universal meter which can readily be altered to change the demand interval length without making extensive mechanical or electrical modifications. One such meter is disclosed by Germer in U.S. Pat. No. 4,179,654, assigned to the assignee of the present invention. The demand register in this meter is capable of being engaged at the start of a demand interval, as determined by a program in the meter, and disengaged to terminate the demand interval as determined by an interval counter generating and end-of-interval signal at a preselected time.
Although an improvement over the prior fixed interval demand meters, the demand intervals occurred at fixed times throughout the day. Since the consumer was able to predict precisely when the demand interval would begin and end, the times of peak power usage could be spread over two demand intervals thereby yielding a lower maximum demand than was actually created. For example, if the user operated at 5 KW for the first half of a fixed period of 15 minutes, at 10 KW for the next half and then remained at 10 KW for the first half of the next fixed period, the fixed period mean maximum would be 7.5 KW instead of the actual 10 KW. In U.S. Pat. No. 4,229,795, Vieweg et al disclosed a device utilizing a shift register and up-down counter to establish a sliding demand period in order to more accurately indicate the actual demand. Although an improvement over the prior art, this device lacks true versatility in that the duration of the sliding period is fixed by the number of stages in the shift register thereby limiting its flexibility since a change in the demand interval duration would apparently necessitate a change in the number of stages of the shift register.
Therefore, a need exists for a time-of-use metering system which provides the capability of readily selecting the time of occurrence and duration of multiple rate periods as well as various lengths of sliding demand intervals without the need for expensive mechanical or electrical modifications to the meter.